The invention relates to a dual-beam spectrometer according to the preamble of the patent claim. Such a spectrometer is composed of at least two entrance slits, a diffraction grating, at least one photodiode array as the optical detector and a collimation lens system. However, the collimation function may also be taken over by a concave grating. Customarily, a holographic concave grating whose diffraction spectrum lies in one plane is employed for this purpose.
Such measuring instruments, called spectral photometers, spectral spectrometers or also array spectrometers are employed in laboratory analysis and in the process measuring art. They can be used to continuously measure gases and liquids, for example, in order to monitor the emission of noxious gases in the exhaust gas of a power plant operated with fossil energy carriers.
Another field of use is high performance liquid chromatography (HPLC), in which the liquid substance mixtures to be examined are split successively in time into their chemical components in a separating column and are converted into electrical signals by a UV-HPLC detector. Such a UV-HPLC detector is composed of a source of ultraviolet light, a flow-through cuvette, a spectrometer and at least one detector array. The division into a measuring beam path and a reference beam path required because of the poor stability of the UV lamp is realized by means of a beam divider.
The periodical Labor Praxis [Laboratory Practice], October, 1988, pages 1091-1094, and the data sheet put out by Milton Roy report of a diode array spectrophotometer "Spectronic 3000 Array" which has two entrance slits, a grating and an array. However, this arrangement is not a dual-beam spectrometer since the two slits do not serve as a measuring slit and a reference slit, respectively. Rather, they permit different wavelengths, visible light and ultraviolet light, to enter the spectrometer separately to then be imaged in succession on one array. The detection of fast light source instabilities is not possible with this system.
Additionally, the periodical Labor Praxis Spezial [Laboratory Practice Special], 1988, pages 90-92, and the data sheet put out by Perkin-Elmer report of an "LC-235" diode array. This system is composed of two superposed entrance slits, a grating and two likewise superposed photodiode arrays. In this way, it is possible to eliminate even very fast lamp fluctuations.
The Proceedings of SPIE, 1988, Vol. 1013, pages 146-153, also describe a spectrometer having several entrance slits which, however, are also superposed on one another so that several arrays must be employed at the output.
U.S. Pat. No. 4,669,873 discloses a spectrophotometer having three entrance slits, a grating and an array. However, with this arrangement only one measuring beam at a time can be analyzed since only one entrance slit is illuminated for a certain period of time.
Of the foregoing measuring systems, those that are able to simultaneously record a measuring spectrum and a reference spectrum and have the entrance slits arranged above one another, employ at least two photodiode arrays or dual-photodiode arrays in which two arrays are accommodated in one housing.
These solutions have the drawback that a stigmatic spectrometer is required for the necessary dot-wise reproduction. Additionally it is necessary to employ a complicated system for separating and coupling the beams into the spectrometer and for separately imaging the beams on the arrays. Such an arrangement is expensive, particularly if dual photodiode arrays are employed.